U.S. patent number 3,602,641 [Application Number 04/860,721] was granted by the patent office on 1971-08-31 for dark current compensation circuit.
This patent grant is currently assigned to U.S. Philips Corporation. Invention is credited to Tom Heise.
United States Patent |
3,602,641 |
Heise |
August 31, 1971 |
DARK CURRENT COMPENSATION CIRCUIT
Abstract
A dark current compensation circuit for a television camera
features a switch for controlling an X-ray tube located in front of
the camera, a video amplifier is coupled to the camera and a peak
rectifier circuit is in turn coupled to the amplifier output. A
switch synchronized with the first switch connects the rectifier to
a capacitor to establish a dark current bias voltage which is
applied to the amplifier.
Inventors: |
Heise; Tom (Emmasingel,
Eindhoven, NL) |
Assignee: |
U.S. Philips Corporation (New
York, NY)
|
Family
ID: |
19804778 |
Appl.
No.: |
04/860,721 |
Filed: |
September 24, 1969 |
Foreign Application Priority Data
|
|
|
|
|
Sep 27, 1968 [NL] |
|
|
6,813,919 |
|
Current U.S.
Class: |
348/243; 348/258;
348/E5.086; 348/E5.078 |
Current CPC
Class: |
H04N
5/217 (20130101); H04N 5/32 (20130101) |
Current International
Class: |
H04N
5/32 (20060101); H04N 5/217 (20060101); H04n
005/32 () |
Field of
Search: |
;178/DIG.5,DIG.26,DIG.29 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3280253 |
October 1966 |
McMaster et al. |
|
Foreign Patent Documents
Primary Examiner: Griffin; Robert L.
Assistant Examiner: Stellar; George G.
Claims
What is claimed is:
1. A circuit for eliminating dark current effects of a television
camera tube, comprising circuit means for alternately causing video
signals and dark current signals to be provided by said tube; a
video amplifier having a signal input coupled to said tube, a bias
input, and an output; a peak rectifier circuit coupled to said
amplifier output; a storage circuit coupled to said bias input; a
first switch coupled intermediate said rectifier circuit and said
storage circuit; said first switch being controlled by said circuit
means to connect said rectifier circuit to said storage circuit
only during the occurrence of said dark current signal and to
disconnect said rectifier circuit from said storage circuit during
the occurrence of said video signals, said circuit means comprising
time-delay means to reconnect the rectifier and storage circuits
through said first switch only after a time delay of at least one
frame period from the cessation of said video signals.
2. A circuit as claimed in claim 1 wherein said storage circuit
comprises a controlled element having an output electrode coupled
to said amplifier and a control electrode; and a first capacitor
coupled to said control electrode.
3. A circuit as claimed in claim 2 wherein said peak rectifier
circuit comprises a second capacitor having a capacitance
substantially equal to 10 times the capacitance of said first
capacitor.
4. A circuit as claimed in claim 1 further comprising means for
generating line-blanking pulses; a gate comprising a controlled
element having an input electrode coupled to said generating means,
and an output electrode coupled to said amplifier bias input, and a
control electrode coupled to said storage circuit.
5. A circuit as claimed in claim 1 wherein said circuit means
further comprises an X-ray tube disposed to irradiate said
television camera tube, and a second switch coupled to said X-ray
tube.
6. A circuit as claimed in claim 1 wherein said time delay
comprises at least two field periods.
7. A circuit as claimed in claim 1 wherein said tube comprises a
photosemiconductive tube.
Description
The invention relates to a device for a television camera tube of
the photosemiconductive type, wherein a given black level upon a
varying dark current in the camera tube is fixed in a picture
signal which is line-and field-generated by the camera tube, which
picture signal has a scanning and a blanking period for each line
and field period, an output of a picture signal amplifier circuit
being connected through a peak rectifier circuit to an input
thereof so as to obtain a varying bias voltage for said amplifier
circuit.
In a camera tube of the photosemiconductive type a scene to be
recorded is projected on a transparent, conducting signal plate
connected through a resistor to a direct voltage source, which
signal plate is provided with a photosemiconductive layer. A
potential image corresponding to the scene is produced on the free
surface of the photosemiconductive layer and is converted into a
picture signal line by line and field by field as a voltage drop
across the said resistor under the influence of an electron beam
scanning the layer and neutralizing the potential image. The
picture signal must be produced between a value corresponding to
black in the scene, or the black level and a value corresponding to
peak white. For a black spot occurring in the scene which spot must
correspond to the black level in the picture signal a minimum
current or the dark current flows in the said resistor, which
current is determined by the leakage current occurring in the
semiconductive layer. It is found that the leakage current in the
photosemiconductive layer and hence the dark current in the said
resistor is generally considerable and has a great temperature
dependence in television camera tubes of the vidicon type. The
result is that the black level in the picture signal produced by
such camera tubes must be set in a variable manner to eliminate the
influence of the dark current variations of the camera tube due to
ambient temperature and voltage variations of the said direct
voltage source on the black level in the picture signal which is
available for further handling.
To carry out the control as described above a device for a
television camera tube of the vidicon type is known from British
Patent Specification 1,045,854. The camera tube applies the
produced picture signal for further handling to a picture signal
amplifier circuit formed as an AC amplifier. The output signal
therefrom is applied during each line-scanning period to a peak
rectifier circuit which measures a black level occurring during the
line-scanning period. During a successive line-blanking period the
measured black level provides a threshold in the peak rectifier
circuit with respect to which the zero value of the picture signal
associated with a blanked electron beam in the vidicon is measured
during the line-blanking period. The discrepancy thus obtained is a
measure of the dark current of the vidicon and is applied as a
variable bias voltage to the input of the amplifier circuit during
the line-blanking period to compensate for the influence of dark
current variations on the black level.
As already stated in the relevant Patent Specification a drawback
of the device described is that the darkest picture element of the
scanned line in the camera tube is measured as the black level
during each line-scanning period and determines the operation of
the device. Since in many scenes the darkest picture element which,
for example, has a grey hue does not correspond to the black level,
it has been proposed to provide an opaque strip on the transparent
screen of the camera tube, so that the black level in the picture
signal produced definitely occurs during each line-scanning period.
In this connection the difficulty has been pointed out that the
opaque strip must be placed in such a manner that the strip is
absolutely not reproduced when reproducing the recorded scene on
the display screen of a television receiver.
Another drawback of providing an opaque strip is that the black
level cannot be measured sufficiently accurately due to influence
of scattered light and the shunt conductance in the
photosemiconductive layer.
It is an object of the present invention to eliminate the said
drawbacks and to provide a device which operates satisfactorily
particularly when making discontinuous use of a continuously
operating television camera tube of the photosemiconductive type.
To this end the device according to the invention is characterized
in that the peak rectifier circuit is coupled to a storage element
through a switch closing after a switching signal, having a time
delay of at least two field periods, which switch is open when the
picture signal comprises video information during the scanning
periods and is closed when video information is absent from the
picture signal, the storage element for providing a bias voltage
dependent on the dark current being coupled to the input of the
picture signal amplifier circuit.
In order that the invention may be readily carried into effect, an
embodiment thereof will now be described in detail, by way of
example, with reference to the accompanying diagrammatic drawing in
which:
FIG. 1 shows an X-ray television circuit employing a device
according to the invention, and
FIG. 2 shows an embodiment of a device according to the
invention.
In FIG. 1 the reference numeral 1 shows a supply source which is,
for example, an AC voltage supply mains. Supply source 1 can be
connected through a bipolar switch 2 to a supply unit 3 which can
apply an alternating voltage, for example, for filament current
purposes and a high direct voltage obtained by transformation and
rectification to an X-ray tube 4. When switch 2 is closed, X-ray
tube 4 can emit X-ray radiation, which is incident on an X-ray
image amplifier 6 after passing through an object 5 such as, for
example, a part of the body of a person to be X-rayed. X-ray image
amplifier 6 converts the amplified X-ray radiation into light which
is projected through a tandem system of lenses 7 onto a television
camera tube 8 in a camera 9. Camera 9 applies a picture signal
produced by camera tube 8 and amplified in a manner not shown to a
cable 10 which leads to a picture signal amplifier circuit 11.
Amplifier circuit 11 applies the picture signal for further
handling to a terminal 12. During further handling the picture
signal can be converted into a video signal which is applied, for
example, to a monitor or into a television signal which is
transmitted by a transmitter for the display of the object 5 on a
television receiver.
An X-ray television circuit is denoted by the reference numerals 2
to 9 inclusive in FIG. 1. As will be apparent from the following,
it would alternatively be possible to describe an infrared
television circuit or a different kind of television circuit in
which the light from a scene is projected in a discontinuous manner
onto a continuously operating camera tube 8.
Only a few components of television camera tube 8 are shown
diagrammatically, such as a transparent, conductive signal plate 13
which is provided with a photosemiconductive layer 14. Signal plate
13 is connected through a resistor 15 to a terminal conveying a
more or less constant variable potential +V. Layer 14 is scanned
line by line and field by field by an electron beam not shown so
that a potential image present thereon and corresponding to the
scene is neutralized. The resultant instantaneous voltage drop
across resistor 15 determines the picture signal applied to the
cable 10. The picture signal comprises two components one of which
must supply the video information between black level and peak
white and the other must supply the dark current flowing through
resistor 15 and corresponding to the unwanted leakage current in
the semiconductive layer 14. For illustration there applies that
the dark current may have approximately the same value as the
current difference between black level and peak white at an ambient
temperature of the camera tube of approximately +20.degree. C. For
an ambient temperature of approximately +40.degree. C. the dark
current may be many times larger, for example, 5 times larger than
the current difference between black level and peak white, which
difference has remained the same in case of an unchanged potential
+V.
To compensate for the influence of dark current variations caused
by ambient temperature variations and variations in the constancy
and the adjustment of the potential +V, the X-ray television
circuit according to FIG. 1 is provided with a device according to
the invention. In the device the picture signal amplifier circuit
11 formed as an AC amplifier applies the picture signal and a pulse
source 16 applies pulses occurring during each line-blanking period
to a clamping circuit 17 incorporated in a peak rectifier circuit
for introducing a direct voltage component into the picture signal.
The peak rectifier circuit is furthermore provided with a
unidirectional current-conducting element formed as a diode 18, a
charge capacitor 19 and a leakage resistor 20. Capacitor 19 may be
connected through a switch 21 to a capacitor 22 employed as a
storage element. The terminal of capacitor 22 connected to the
switch 21 is connected through a gating circuit 23, to which
switching pulses of line frequency are applied by the pulse source
16, to the picture signal amplifier circuit 11 for supplying a bias
voltage. As is shown by a broken line, the switches 2 and 21 are
coupled in a mechanical or electromechanical manner through a time
delaying element 24 with a time delay .tau.. Initially switch 2 is
open and switch 21 is closed, when switch 2 is closed switch 21 is
opened at the same time, while subsequent opening of switch 2
causes the switch 21 to be again closed only after a time
delay.
The operation of the X-ray television circuit according to the
invention is as follows: initially when switch 2 is open, during
which switch 21 is closed, the X-ray tube 4 is switched off, so
that no light is projected onto the screen of the television camera
tube 8. The result is that the picture signal produced by the
camera tube does not comprise video information, but only
represents the dark current. Starting from a positively directed
picture signal supplied by circuit 11 at which the dark current
during the line-scanning period is positive relative to the value
of the picture signal during the line-blanking period, the
capacitors 19 and 22 will convey a voltage which corresponds to the
peak value of the dark current. Capacitor 22 applies through the
gating circuit 23 a proportional bias voltage to the picture signal
amplifier circuit 11. If the value of the dark current changes, for
example, due to a variation of the ambient temperature of the
camera tube 8, the bias voltage applied to the circuit 11 will also
change through the peak rectifier circuit (17-20), switch 21,
capacitor 22 and gating circuit 23 in such a manner that the level
corresponding to the dark current in the picture signal provided by
the picture signal amplifier circuit 11 does not change. Since the
dark current component and the black level must coincide in a
picture signal which comprises both a video and a dark current
component the result would be that the black level would be set
when video information is present.
The closing of switch 2 and the concurrent opening of switch 21
results in the X-ray tube 4 being activated. The object 5 is
consequently displayed on the screen of the camera tube 8, so that
the picture signal produced by the tube 8 comprises video
information. The picture signal amplifier circuit 11 applies the
picture signal comprising a positively directed video component to
the peak rectifier circuit (17-20) so that capacitor 19 is charged
to the maximum value of the picture signal. Since the switch 21 is
open, the voltage across capacitor 19 cannot influence the constant
voltage across capacitor 22, so that the bias voltage across the
amplifier circuit 11 and corresponding to the dark current does not
change. The X-ray examination of the object 5 which may be, for
example, a part of a body of a person must be performed as quickly
as possible so as to prevent injury thereof, so that after closing
switch 2, it is reopened after a short time.
Opening switch 2 manually or automatically renders the X-ray tube 4
inoperative. Since the line and field scanning in the camera tube 8
continues the potential image on the photosemiconductive layer 14
will be neutralized after two fields such that the video component
no longer occurs in the picture signal produced by the camera tube
8. Subsequently the charge across capacitor 19 which is determined
by the maximum value of the picture signal including the video
component will leak away through resistor 20. Dependent on the
RC-time constant, capacitor 19 will again obtain the charge after a
short time which corresponds to the dark current. At that instant
which may be after a duration .tau. after switching off switch 2,
switch 21 can be closed. If the dark current has changed during the
period of X-ray examination the voltage across capacitor 22 will
become equal to the changed voltage across capacitor 19. The result
is that the bias voltage applied to the amplifier circuit 11 is
adapted to the changed dark current.
The capacitance of capacitor 22 must be small relative to that of
capacitor 19, so that in case of a higher voltage across capacitor
19 this capacitor is not too heavily loaded by the capacitor 22,
while in case of a smaller voltage across capacitor 19 it must be
possible for capacitor 22 to follow it quickly. In an embodiment of
the device a ratio of 1 to 10 is found to be satisfactory.
It appears in practice that the time delay .tau. for an X-ray
television circuit must be several seconds. This is caused by the
fact that after switching off switch 2, the X-ray radiation does
not immediately drop out under the influence of the high voltage
which is still applied for some time by the supply unit 3 to the
X-ray tube 4. For measuring the dark current in a very accurate
manner, a time delay .tau. of approximately 20 seconds was found to
be sufficient. Even in a plurality of examinations successively
performed at waiting periods shorter than 20 seconds, the dark
current variation was found to vary so slowly that a satisfactory
stabilization of the black level in the picture signal supplied by
the picture signal amplifier circuit 11 was still ensured.
FIG. 2 shows a detailed embodiment of a device according to the
invention. The device may be used, for example, in X-ray infrared
television circuits and similar circuits.
Components already having reference numerals in FIG. 1 are denoted
by the same reference numerals in FIG. 2, although in case of an
unchanged function the manner of connection in the device of FIG. 2
may be modified.
The cable 10 of FIG. 1 is divided into two lines 10.sub.1 and
10.sub.2 in FIG. 2 which apply a picture signal 25 produced by
camera tube 8 and amplified in the camera 9 to the picture signal
amplifier circuit 11. The picture signal 25 is shown by picture
signals 25.sub.1 and 25.sub.2 shown by solid and broken lines,
respectively the significance of which will be apparent from the
following. The positively directed picture signal 25 is shown for
one and a half line periods, the line-blanking period being denoted
by T.sub.b and the line-scanning period being denoted by T.sub.s.
Starting from a relationship which is common for X-ray television
the line-blanking period T.sub.b is slightly shorter and the
line-scanning period T.sub.s is slightly longer than half a line
period. The picture signal 25.sub.1 shown by solid lines may be
produced in a camera tube 8 of the vidicon type the ambient
temperature of which is approximately 20.degree. C. As a result a
picture signal 25.sub.1 is shown the video and dark current
components of which are equally large during the line-scanning
period T.sub.s. The video component is shown by an obliquely
directed line which is considered to vary from the black level in
the middle of one line period to peak white at the end thereof. The
difference between the black level shown and the level shown in the
line-blanking period T.sub.b indicates the value of the dark
current. For, example, a higher ambient temperature of the camera
tube 8 the dark current will increase which is indicated in the
picture signal 25.sub.2 shown by broken lines for a dark current
which is twice as large.
The lines 10.sub.1 and 10.sub.2 of cable 10 are connected together
through a resistor 26 which is characteristic for the cable. The
line 10.sub.2 is connected through an isolation capacitor 27 to a
terminal of a capacitor 28 the other terminal of which is connected
to ground, and to one end of resistor 29 and 30, the other ends of
which are connected to an emitter electrode of a transistor 31 and
to a terminal conveying a constant voltage -V.sub.2, respectively
The terminal conveying the voltage -V.sub.2 forms part of a supply
source V.sub.2 not shown, whose terminals conveying the voltages
+V.sub.2 and -V.sub.2 will hereinafter be referred to as terminals
+V.sub.2 and -V.sub.2. The line 10.sub.1 is connected through an
isolation capacitor 32 to the base electrode of the transistor 31.
The base electrode of transistor 31 is connected through a resistor
33 and a resistor 34 to earth and to the capacitor 28,
respectively. The collector electrode of the transistor 31 is
connected through a resistor 35 to the terminal +V.sub.2 and is
connected to a base electrode of a transistor 36 arranged as an
emitter follower. The emitter electrode of transistor 36 is
connected to the terminal 12. Transistors 31 and 36 are the active
components which form part of the picture signal amplifier circuit
11.
Since the mean values in the picture signals 25.sub.1 and 25.sub.2
are unequal the result without taking further steps would be that
the picture signal amplifier circuit 11 formed as an AC amplifier
would supply voltages by which, when being applied to a clamping
circuit active during the line-blanking period T.sub.b, the black
level in only one of the two voltages would be determined, as the
levels have a different value in both signals.
In the device of FIG. 2 according to the invention clamping circuit
17 is formed with a transistor 37 to the base electrode of which
pulses 38 occurring during the line-blanking period T.sub.b and
supplied by the pulse source 16 are applied, while the emitter
electrode is connected to a tapping or a potentiometer 39 which is
connected between the terminals -V.sub.2 and +V.sub.2. The tapping
on potentiometers 39 is connected through a capacitor 40 to the
terminal -V.sub.2. The collector electrode of transistor 37 is
connected through a capacitor 41 to terminal 12 and is also
connected to a base electrode of a transistor 42 which is connected
as an emitter follower through a resistor 43 to the terminal
-V.sub.2. The emitter electrode of transistor 42 is connected to
the cathode of the diode 18, whose anode is connected both to a
terminal of the capacitor 19, whose other terminal is connected to
earth, and through the leakage resistor 20 to the terminal
+V.sub.2. The voltage conveying terminal of capacitor 19 may be
connected to a terminal of capacitor 22 with the aid of switch 21
and the time-delaying element 24 through a smoothing resistor 44.
The other terminal of capacitor 22 is connected to earth. The
voltage conveying terminal of capacitor 22 is connected to a
control electrode G of a field effect transistor 45 which is active
as a voltage-controlled unidirectional current-conducting element.
An electrode D of the field effect transistor 45 is connected to
the terminal +V.sub.2 and an electrode S is connected through a
resistor 46 to the terminal -V.sub.2. The electrode S serves as an
output electrode and is connected to a base electrode of a
transistor 47. The emitter electrode of transistor 47 is connected
through a resistor 48 to the pulse source 16 which supplies pulses
49 during the line-blanking period T.sub.b. The collector electrode
of the transistor 47 active in the gating circuit 23 is connected
to the emitter electrode of the transistor 31 in the picture signal
amplifier circuit 11.
To explain the operation of the device according to the invention,
the starting point is a device in which switch 21 is open and
capacitor 22 has obtained a voltage at a previous closure of switch
21, which voltage is associated with the dark current component in
the picture signal 25.sub.1. The constant voltage across capacitor
22 having a value of, for example, +3V yields a constant current
flowing through resistor 46 via the field effect transistor 45, so
that a constant voltage of, for example, +6V is impressed on the
base electrode of transistor 47. The pulses 49 which are, for
example, between approximately + 6.5 V and 0V cutoff transistor 47
during the negatively directed pulses which have a duration of, for
example, one fourth to approximately three fourths of the
line-blanking period T.sub.b. The transistor 47 is bottomed outside
this range. The result is that a current I indicated by I.sub.1
will flow in the emitter collector circuit of transistor 47 in
which the resistors 48, 29 and 30 and the capacitor 28 are
arranged. As a result the current I.sub.1 flowing through resistor
29 will supply an additional voltage during the period of the
transistor 47 being bottomed, so that the negative voltage across
the capacitor 28 becomes less negative. When transistor 47 is
cutoff capacitor 28 only determines the bias voltage which is
impressed on the emitter electrode of transistor 31. The result is
that the collector electrode of transistor 31 conveys a voltage 50
indicated by 50.sub.1 which is plotted relative to a level +V.sub.2
for the purpose of illustration. If the control through transistor
47 would not be used, voltage 50.sub.1 would be uniform in phase
opposition to the picture signal 25.sub.1 and would be so much
lower with respect to the level +V.sub.2 as corresponds to the
height of the pulses in the voltage 50.sub.1.
The voltage 50.sub.1 is applied in a uniform manner to the cathode
of the diode 18 through the transistors 36 and 42 connected as
emitter followers and after the introduction of a direct voltage
component with the aid of clamping circuit 17. Capacitor 19 is
therefore charged to its highest value in the negatively directed
voltage 50.sub.1, for example, peak white.
As described in FIG. 1, a video component will not any longer occur
in the picture signal 25.sub.1 and hence voltage 50.sub.1 after
opening switch 2 and a delay time .tau. of the time-delaying
element 24 so that a sufficiently discharged capacitor 19 will
convey a voltage (+ 3V) which corresponds to the dark current
component. Apart from leakage losses for capacitor 22, the voltages
across the capacitors 19 and 22 will be equal for a dark current
component which is unchanged relative to the previous measurement
so that no charge transport takes place when switch 21 closes.
A similar explanation as the one described above applies to the
picture signal 25.sub.2. Due to the larger dark current component
the voltage across capacitor 19 will be less positive, for example,
+2V when the video component is absent which voltage is impressed
through switch 21 upon capacitor 22. The voltage impressed on the
base electrode of transistor 47 will therefore likewise be less
positive, for example, +5V, so that a current I.sub.2 flows which
is larger than I.sub.1 due to the constant amplitude of the pulses
49. The result is that the collector electrode of the transistor 31
conveys a voltage 50.sub.2 under the influence of the larger
voltage drop across resistor 29 caused by the current I.sub.2. An
additional direct voltage component is introduced by a larger
current I.sub.2 with the aid of transistor 47, which component
relative to the current I.sub.1 corresponds to the height of the
pulses in the voltage 50.sub.2 shown by broken lines.
For illustration the level +V.sub.2 is shown for the voltages
50.sub.1 and 50.sub.2. It is evident that the level +V.sub.2 should
be much higher for a dark current component which may be, for
example, 5 times higher than that in the picture signal
25.sub.1.
To be able to measure small dark current variations, the
amplification which can be obtained with the aid of the picture
signal amplifier circuit 11 must be as large as possible.
It will be evident that for obtaining the smallest possible losses
in the device, it is required that transistor 47 is not bottomed
when the smallest possible dark current component occurs in the
picture signal 25. Transistor 47 would have to convey its maximum
current at the greatest dark current to be expected. For
illustration there applies that the voltage impressed on the base
electrode of transistor 47 may vary from +6V to 0V for a pulse
value of, for example, between +6V and 0V of the pulses 49, the
voltages across capacitors 19 and 22 varying, for example, from +3V
to -3V in case of a change from the smallest to the largest dark
current.
A few values of components important for an embodiment of the
device follows for the purpose of illustration.
C.sub.19 =10 .mu.f.
c.sub.22 =1 .mu. f.
r20= 100 k.Ohm.
R44= 100 k.Ohm.
R29= 270 Ohm.
R48= 270 Ohm.
* * * * *